The term navigation can be broadly defined as the means by which a craft is given guidance to travel from one known location to another. Irrespective of the size, speed or type of craft, or duration of travel, arrival at the desired destination is of paramount importance. Historically, a variety of methods have been invoked in order to accomplish navigation of a moving vehicle. With the advent of satellite position determination systems, such as the NavStar GPS, operated by the U.S. Government, precise position determination within a few meters can be accurately determined and a travel path interpolated by virtue of recording several such data points. Thus, in most instances GPS type systems have become a preferred method of navigating position determination.
The GPS system comprises a constellation of earth orbiting space vehicles that continuously transmit dual frequency telemetry that provide timing information relative to the specific spacecraft. A user having a GPS receiver, tunable to either of the two provided frequencies (L1 or L2) is able to accurately determine his position relative to the earth's surface by virtue of acquiring the signals from four or more spacecraft and determining the range of the receiver from each craft. Well known processing techniques are then utilized in order to provide the user meaningful longitude, latitude and altitude information. One of the two codes utilized, precise code or P-code, has an exceptionally long data sequence modulated at 10.23 MHz. The other telemetry stream is referred to as course/acquisition mode or C/A-code and is a gold code sequence having a chip rate of 1.023 MHz. The gold-code sequence is a well known conventional pseudo-random sequence repeated once every millisecond. Positional accuracy between the two codes varies greatly with the P-code providing the more precise measurement. In order to increase the accuracy of the C/A-code many GPS receivers work in combination with an additional signal emanating from a known location or source, thereby providing accuracy equal to or exceeding P-code. This approach is generally referred to as differential GPS, and is well known in the art.
Initial GPS receivers were quite large and expensive thereby mandating user platforms to relatively large land based or airborne vehicles. As a result of the continued evolution in the electronics industry, the physical size of individual components has shrunk exponentially, while simultaneously exhibiting functional increases of vast proportion. As a direct result of such electronic evolution, the unit price of good quality GPS receivers has continued to decrease such that prices less than five hundred dollars per unit are rather typical. The attainment of relatively low receiver unit pricing affords GPS technology to many applications heretofore considered inappropriate or non-cost beneficial for such applications.
Unmanned vehicles whether self-propelled or projected, represent a new arena of items that could benefit from GPS technology. Whether utilized in a manner to provide control and guidance during the course of flight, or whether the results of a first flight is utilized for adjustments in the trajectory of subsequent items, the inclusion of miniaturized GPS receivers on or in such items provides benefits heretofore not available to such users.
By way of example, large artillery shells directed at medium range targets (30-50 kilometers) determine target distance by virtue of a "registration" shell fired at the target. The registration shell that contains a portion of a GPS receiver and a data translator, which in effect transmits the received GPS data over a wideband signal to a ground user at a remote location. Unfortunately, a wideband analog translator is typically easily detected by surrounding users, contains no additional telemetry capability, and by virtue of size and power requirements is limited to a single frequency. Special ground equipment is required in order to receive the repeated signal from the registration shell, and in general, the system cannot accommodate more than one registration shell in flight at a given time. The accuracy provided the user of such system is generally considered to warrant the increased risk by minimizing the number of attempts necessary to place an object in a given location, and the corresponding time period from beginning to end of the mission. However, improvements in the prior art system which would provide increased stealth capability, accommodate telemetry down-link and the providing of control signals to the shell would be advantageous. Additionally, the minimization of any gear required to support such operations would also be of great benefit.
A new class of munitions, referred to as "competent" munitions, has been developed to further the end of improving the accuracy of traditional type munitions at a relatively low cost and complexity. Competent munitions are a branch of artillery munitions situated between traditional "dumb" munitions and "smart" munitions. A "dumb" munition is an area weapon incorporating no accuracy improvements. A "smart" munition is a point weapon incorporating a seeker or similar capability that seeks a particular target or target profile and navigates to that point. Thus, a "smart" munition may utilize GPS navigation as a targeting and guidance system, for example. As the range of artillery munitions increases from traditional ranges of 20-25 kilometers out to 50 kilometers and even 100 kilometers, the accuracy of "dumb" munitions degrades to a level that is unacceptable. Thus the "competent" munition is a recent innovation intended to regain the accuracy needed to make a munition accurate enough for area weapon missions at these extended ranges, while maintaining low costs close to those of "dumb" munitions. Additionally, there is some desire to use certain highly accurate competent munitions as pseudo-point weapons against large point targets that are hard to seek such as bunkers.
The general categories for competent munitions include auto-registration, range-only correction (one-dimensional), and fully-guided (two-dimensional) correction. Auto-registration, or self-registration, involves the shell providing information about its location while in flight to allow an offset from the expected or desired impact point to be calculated. This offset is used to more accurately point the gun to improve the accuracy of subsequent rounds. Auto-registration is typically the least expensive competent munition, and only the registration rounds need to have an auto-registration fuze on them, further reducing the ost of this capability. It can generally provide a two to three times error reduction over a "dumb" munition out to ranges of about 40 kilometers.
Range-only correction includes on-board navigation capability only to the level required to correct the range of the munition, correction in a single dimension. In this type of shell, the round is purposely fired at a point disposed beyond the target and a drag device is deployed at a predetermined point in the trajectory of the munition. By controlling the timing of deployment of the drag apparatus and even the amount of drag, an accurate range can be achieved. Thus, the trajectory of a shell that would nominally overshoot the target location is altered in flight by a drag apparatus thereby redirecting the shell tc impact the target location. Since this shell includes on-board correction, all rounds must have the self-correcting fuze to take advantage of this capability. This fuze is probably only marginally more expensive than the auto-registration fuze, however. The range error is much larger than the cross track er-or, so just correcting the range can yield a three to six times error reduction over a "dumb" munition out to ranges of 50 or more kilometers.
Fully-guided (two-dimensional in flight correction) shells include on-board navigation to correct both the range and cross-track errors of the munition as it travels along its nominal trajectory. This type of shell requires on-board inertial sensors, as well as a complete navigation and mechanical steering capability. The accuracy of the shell is thus nearly always the same regardless of range. Since this shell includes on-board correction, all rounds must have the self-correcting fuze to take advantage of this capability. This fuze is expected to be considerably more expensive than the range-only corrected or auto-registration fuze. It can yield a three to more than ten times error reduction over a "dumb" munition out to ranges of 100 kilometers.
Since range-only correction requires a complete GPS receiver on-board the shell, the addition of an auto-registration capability only very slightly increases the circuit volume required and the cost of the fuze. This capability can then provide the range accuracy improvements of range-only correction combined with the cross track accuracy improvements of auto-registration. The range correction plus auto-registration round is expected to provide accuracies approaching that of fully-guided munitions without requiring complete inertial, navigation, and mechanical steering systems onboard the shell.
Accordingly, a need exists for an improved method of navigating and monitoring the results of such navigation of unmanned apparatus such as high-velocity missiles or shell projectiles. There further lies a need for a remote position sensing and control system for use with an unmanned flight vehicle which provides a combination of auto-registration and range correction.